1. Field of the Invention
The present invention relates to a pneumatic tire provided with, on a tread face, a shoulder land portion extending in a circumferential direction of the tire, and a narrow groove extending in the circumferential direction of the tire near a ground contact end of the shoulder land portion and dividing the shoulder land portion into a main body land portion on the inner side in a width direction of the tire and a sacrificed land portion on the outer side in the width direction of the tire.
2. Description of the Related Art
In order to prevent irregular wear of a tire, a technique for providing, on a tread face, a narrow groove near a ground contact end of a shoulder land portion extending in a circumferential direction of the tire has been conventionally, publicly known. However, when a sacrificed land portion is excessively deformed by an input from a road surface while running, strain is concentrated on a groove bottom so as to generate a crack. Thus, there is a problem on durability of a tire.
For example, as shown in FIG. 3, especially when a vehicle runs on the stepped portion 10, such as a shoulder of a road, extending in a vehicle running direction, the sacrificed portion 11 is excessively deformed by overriding on the stepped portion 10. Further, when a groove bottom of a narrow groove 12 receives an input in the arrow direction, a crack of such a groove bottom easily occurs. In this case, when the sacrificed land portion 11 is made thinner, stress hardly works on the groove bottom even when the sacrificed land portion 11 is deformed, and thus a crack hardly occurs. However, there are problems that the sacrificed land portion 11 itself may be torn and removed, and the prevention effect of irregular wear of a tire decreases.
As means for preventing generation of a crack in a narrow groove, for example, Japanese Unexamined Patent Publication (Kokai) No. Hei 11-301214 discloses a technique for preventing generation of a crack at a groove bottom of a narrow groove or a portion near the groove bottom by providing, in the middle of a depth direction of the narrow groove and at an inner wall of the narrow groove in the main body land portion side at least one enlarged portion continuing in a circumferential direction of the tire. However, as for the pneumatic tire mentioned above, when a sacrificed land portion receives an input from a road surface and falls toward the main body land portion, the main body land portion is easily deformed, and thus the main body land portion is worn in comparative-early time. Thus, there arises a problem that an object of providing the narrow groove, i.e., retarding wear of the main body land portion so as to prevent irregular wear, cannot be realized. Further, although the enlarged portion is provided at the groove bottom, a main object of a form of the narrow groove in the pneumatic tire in Japanese Unexamined Patent Publication (Kokai) No. Hei 11-301214 is to reduce strain caused by deforming the main body land portion. Thus, Japanese Unexamined Patent Publication (Kokai) No. Hei 11-301214 does not disclose a technique for preventing a crack caused by deforming the sacrificed land portion. Furthermore, this technique cannot prevent a crack in a narrow groove when receiving the big input from the stepped portion 10 as shown in FIG. 3.
Further, Japanese Unexamined Patent Publication (Kokai) No. Hei 7-76204 discloses a technique for reducing generation of a crack at a groove bottom by providing at least one cut groove extending in a circumferential direction of a tire at an inner wall on the main body land portion side of the narrow groove, and by making an area of the inner wall on the main body land portion side to be larger than an area of an inner wall on the sacrificed land portion side. However, as for the pneumatic tire mentioned above, the main body land portion is easily deformed since the cut groove is provided at an inner wall on the main body land portion side. Therefore, there is a similar problem in Japanese Unexamined Patent Publication (Kokai) No. Hei 11-301214.
Further, Japanese Unexamined Patent Publication (Kokai) No. Hei 6-191230 discloses a technique for preventing generation of a crack at a narrow groove caused by biting a stone, by providing, at an intermediate portion in a depth direction of the narrow groove, an enlarged portion enlarging in a width direction of a tire. However, as for this pneumatic tire, generation of a crack is prevented by preventing biting a stone in the narrow groove, therefore generation of a crack caused by the input from a road surface mentioned above cannot be prevented.
Furthermore, Published Japanese Translation No. 2002-512575 of the PCT Application discloses a technique for preventing generation of a crack at a groove bottom by providing an enlarged portion on the sacrificed land portion side of the groove bottom in a narrow groove. However, when a sacrificed land portion is excessively deformed at the time of particularly running on a shoulder of a road or the like, so as to work a large stress on the groove bottom, an effect to distribute the stress is insufficient, and thus generation of a crack at the groove bottom may not be prevented.